Pydantic Stealth breaking change 2.9 -> 2.10 for type

The Fix

pip install pydantic==1.10.19

Based on closed pydantic/pydantic issue #11853 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -767,6 +767,12 @@ def _unpack_refs_defs(self, schema: CoreSchema) -> CoreSchema:
         return schema
 
+    def _resolve_self_type(self, obj: Any) -> Any:
+        obj = self.model_type_stack.get()
+        if obj is None:

repro.py

class Endpoint(BaseModel):
    endpoint_func: Callable
    input_schema: INPUT_SCHEMA
    output_schema: OUTPUT_SCHEMA

    @field_validator("output_schema", mode="plain")
    @classmethod
    def validate_output_schema(cls, v: OUTPUT_SCHEMA) -> OUTPUT_SCHEMA:
        ...

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install pydantic==1.10.19\nWhen NOT to use: This fix should not be applied if the code relies on the deprecated behavior of type subscripting.\n\n

Why This Fix Works in Production

Trigger: pydantic.errors.PydanticUserError: Subscripting `type[]` with an already parametrized type is not supported. Instead of using…
Mechanism: Addresses issues related to the usage of `type` with `typing.Self` and type aliases, fixing errors that arose from subscripting `type[]` with already parametrized types.
Why the fix works: Addresses issues related to the usage of `type` with `typing.Self` and type aliases, fixing errors that arose from subscripting `type[]` with already parametrized types. (first fixed release: 1.10.19).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Surfaces as: pydantic.errors.PydanticUserError: Subscripting `type[]` with an already parametrized type is not supported. Instead of using type[typing.Sequence[pydantic.main.BaseModel]], use…

Proof / Evidence

GitHub issue: #11853
Fix PR: https://github.com/pydantic/pydantic/pull/10621
First fixed release: 1.10.19
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.62

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“typing.Sequence is a deprecated alias to collections.abc.Sequence. This alias is implemented as an instance of an internal typing construct and as such doesn't pass the…”

@Viicos · 2025-05-15 · source

“@Viicos Thanks for the quick reply! I applied your suggestions - removed SkipValidation and changed the validators to "plain"”

@JanSobus · 2025-05-11 · source

“An extra check was introduced in 2.10 by https://github.com/pydantic/pydantic/pull/10621”

@Viicos · 2025-05-09 · source

Failure Signature (Search String)

pydantic.errors.PydanticUserError: Subscripting `type[]` with an already parametrized type is not supported. Instead of using type[typing.Sequence[pydantic.main.BaseModel]], use

Error Message

Stack trace

error.txt

Error Message
-------------
pydantic.errors.PydanticUserError: Subscripting `type[]` with an already parametrized type is not supported. Instead of using type[typing.Sequence[pydantic.main.BaseModel]], use type[Sequence].

Minimal Reproduction

repro.py

class Endpoint(BaseModel):
    endpoint_func: Callable
    input_schema: INPUT_SCHEMA
    output_schema: OUTPUT_SCHEMA

    @field_validator("output_schema", mode="plain")
    @classmethod
    def validate_output_schema(cls, v: OUTPUT_SCHEMA) -> OUTPUT_SCHEMA:
        ...

Environment

Pydantic: 2

What Broke

Users experienced type errors when defining models that previously worked in version 2.9.

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==1.10.19

When NOT to use: This fix should not be applied if the code relies on the deprecated behavior of type subscripting.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pydantic/pydantic/pull/10621

First fixed release: 1.10.19

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be applied if the code relies on the deprecated behavior of type subscripting.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
1.10.19	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic Stealth breaking change 2.9 -> 2.10 for type sequences (Fix)